The invention relates to a reaction mass containing mixed oxides of iron with one or more metals of groups IV to VII of the periodic table of the elements, a method for the manufacture of such reaction mass and the use thereof for the shift reaction and/or the removal of sulphur compounds from gases.
The removal of sulphur compounds from gas mixtures and the conversion of the sulphur compounds removed to elementary sulphur which can easily be stored and further processed into valuable products, is of extraordinary technical significance. In the production of refined fossil fuels, the sulphur is present in the form of hydrogen sulphide, carbon disulphide or carbonyl sulphide. Many valuable natural gases contain noticeable proportions of hydrogen sulphide. This leads to undesirable air pollution when these gases are burnt. In order to avoid this air pollution, the sulphur compounds must be removed from the gases prior to combustion. Removal of sulphur compounds is particularly easy to carry out when solid regenerable absorbent substances are used.
In order to desulphurise crude oil, the crude oil fractions are treated catalytically with hydrogen. The catalytic reaction converts the sulphur to hydrogen sulphide, which must then be separated from the hydrogen, in order to be able to return the latter in the process. After separation, the hydrogen sulphide is in general converted to elementary sulphur by the Claus method.
To reduce air pollution by combustion of coal, the coal can be converted to liquid or gaseous fuels which are then refined. Liquid fuels obtained from coal can be refined in the same way as crude oil fractions. The hydrogen sulphide obtained can be separated and further processes as described. Coal gasification with steam, forming hydrogen and carbon monoxide, is technically very attractive. This conversion has been carried out for decades, producing so-called water gas which is then further processed into consumer gas. During conversion of the coal to hydrogen and carbon monoxide, either the sulphur compounds remain in the inorganic solids obtained as residue (mainly sulphates), or they are converted to hydrogen sulphide, carbon disulphide or cabonyl sulphide. In any case the sulphur compounds must be removed from the gas mixture.
The mixture of carbon monoxide and hydrogen obtained in the gasification of coal can be burnt after purifying without risk of air pollution by sulphur dioxide. Since the presence of the highly toxic carbon monoxide is not always permitted in consumer gases, the gas mixture must in many cases be further processed. The carbon monoxide/hydrogen mixture which is known as synthesis gas may be further processed catalytically, producing methanol, methane or liquid hydrocarbons. Such catalytic processes, however, require thorough removal of the sulphur compounds since the catalysts used are readily poisoned and deactivated by sulphur or sulphur compounds. The same problems arise when synthesis gas (carbon monoxide/hydrogen) which is to be converted to methanol or ammonia is manufactured from natural gas or crude oil fractions. The natural gas or the gasified crude oil fractions can be converted with steam in the presence of substrate-containing nickel catalysts to carbon monoxide and hydrogen ("steam reforming process"). In this case as well the natural gas or the light crude oil fractions must be freed from sulphur compounds, as sulphur or sulphur compounds poison the nickel steam reforming catalysts. Heavier crude oil fractions are reacted with oxygen and steam at higher temperatures (so-called high-temperature partial oxidation process). In this case, the gas mixture obtained by this method must be purified.
From the above statements it is evident that the removal of sulphur compounds from gas mixtures, in particular reducing gas mixtures, is of the greatest importance. According to the prior art, in general the hydrogen sulphide is removed from reducing gases by physical absorption. This method is associated with various difficulties. Apart from the extensive installations required and the large quantities of valuable organic solvents which are necessary therefor, there is the particular disadvantage technically that the gas mixture must be cooled to temperatures of down to below 100.degree. C. Moreover, at the same time absorption of carbon dioxide takes place, which is difficult to avoid. The industrial catalytic processes following thereafter are, on the other hand, carried out at high temperatures of at least approximately 200.degree. C. or even higher temperatures, so that the gases must therefore be reheated after desulphurisation. When the hydrogen sulphide is removed by desorption from the fluids by heating, in general it is converted by the Claus method to elementary sulphur. By this method, oxidation of sulphur is carried out in two stages for reasons of kinetics and thermodynamics. At low temperatures the equilibrium reaction EQU 2H.sub.2 S+O.sub.2 .fwdarw.2S+2H.sub.2 O
is favourable. The rate of reaction is low, however, and the heat of reaction causes an increase in temperature. Therefore several reactors with intermediate cooling must be used. For this reason it is more advantageous to convert hydrogen sulphide at higher temperatures to sulphur dioxide according to the following equation: EQU 2H.sub.2 S+3O.sub.2 .fwdarw.2SO.sub.2 +2H.sub.2 O
Even at high temperatures, equilibrium lies markedly on the right-hand side. The heat of reaction can be used to heat up other products. If the H.sub.2 S content in the gas is below about 30 vol.%, however, it is difficult to ignite the hydrogen sulphide-air mixture. In particular when carbon dioxide is absorbed simultaneously with the hydrogen sulphide, appreciable difficulties arise with the Claus method.
Sulphur dioxide is reacted at lower temperatures on solid catalysts (generally activated aluminium oxide) with the remaining portion of hydrogen sulphide according to the following equation: EQU SO.sub.2 +2H.sub.2 S.fwdarw.3S+2H.sub.2 O
As the ratio of H.sub.2 S to SO.sub.2 must be kept to 2, this method is sensitive to changes in the proportions of gases in the gas stream to be processed. Start-up of the Claus process is difficult for this reason as well.
With regard to the above difficulties, it has been proposed that solid absorbents be used to remove sulphur compounds. From German Offenlegungsschrift No. 21 44 567 is known a desulphurising mass which contains divalent copper oxide on a porous substrate. The copper oxide is in relatively large particles, and regeneration of this absorbing mass is very eleborate. In German Offenlegungsschrift No. 31 31 257, to remove sulphur compounds there is proposed an absorbing mass which contains metal oxides on an inert refractory substrate which has a specific surface area of more than 10 m.sup.2 per g, wherein the substrate is charged with the metal oxide in a quantity of at least 5 wt.%, calculated as metal of the active component and based on the weight of the substrate, and wherein at least 50 wt.% of the metal oxides on the substrate are in finely divided form with a particle size of less than 40 nm. Particularly iron oxide is very effective as the active metal oxide. Absorbent masses which contain such finely divided iron oxide can absorb hydrogen sulphide from reducing gas mixtures up to a sulphur-to-iron atomic ratio of about 1. At a temperature of 500.degree. C., absorption capacity is generally greater than at 300.degree. C., at which temperature an atomic sulphur to active metal ratio of about 0.7 can be obtained.
For reasons of better utilisation of heat, desulphurisation at elevated temperatures is very advantageous. Equilibrium for conversion to hydrogen sulphide EQU CS.sub.2 +H.sub.2 O.fwdarw.COS+H.sub.2 S
and EQU COS+H.sub.2 O.fwdarw.CO.sub.2 H.sub.2 S
is also more advantageous at higher temperatures. Separate hydrolysis of carbon disulphide and carbonyl sulphide is therefore not necessary. The aforementioned compounds are, on the contrary, converted to hydrogen sulphide during the desulphurisation reaction. As the subsequent catalytic reactions are generally carried out at 300.degree. C. or even higher temperatures, additional heating up of the purified gas mixture after high-temperature desulphurisation is generally not necessary.
A further essential advantage of the aforementioned absorbent mass containing finely divided iron oxide lies in that on regeneration of the saturated absorbent mass with oxygen, elementary sulphur is obtained. Therefore the extensive and costly equipment of the Claus process and of waste gas treatment can be avoided. In the aforementioned German Offenlegungsschrift No. 31 31 257 there is described in detail how the formation of sulphur dioxide can be avoided and the rate of regeneration can be optimised.
Although according to the aforementioned German Offenlegungsschrift excellent absorbent masses are described, there is naturally a need to find even better absorbent masses, in particular those with a greater absorption capacity. With higher absorption capacity, smaller equipment can be used for desulphurisation, leading to a reduction in investment costs. A further advantage of a higher absorption capacity is that for the same absorption-regeneration cycle time, a smaller quantity of reaction mass is sufficient. The pellets in the lower part of the reactor are then subjected to less mechanical stress. The present invention is therefore in particular based on the object of finding reaction masses which can be used with excellent results to remove sulphur compounds from gases and have a higher absorption capacity for the sulphur compounds.
The carbon monoxide shift reaction is the reaction of carbon monoxide with steam, forming carbon dioxide and hydrogen. This reaction must as a rule be carried out to adjust suitably the hydrogen-to-carbon monoxide ratio of synthesis gas for further processing. For conversion to methane, for example, a hydrogen-to-carbon monoxide ratio of 3 or more is required, in order for the following reaction to take place: EQU 3H.sub.2 +CO.fwdarw.CH.sub.4 H.sub.2 O
The carbon monoxide shift reaction EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
requires excess steam in order to shift the equilibrium sufficiently to the right. It is therefore advantageous if one is in a position to combine the carbon monoxide shift reaction with desulphurisation, or to carry out the carbon monoxide shift reaction prior to removal of sulphur. The steam which is added for the reaction with the coal, wherein the steam-to-carbon ratio may amount to up to 4, can be used subsequently in the carbon monoxide shift reaction which is carried out at a lower temperature than coal gasification. A combination of desulphurisation and the carbon monoxide shift reaction is particularly advantageous as the required plant can be designed very much smaller. The present invention is therefore also based on the object of finding a reaction mass which catalytically accelerates the carbon monoxide shift reaction.